1. Field of the Invention:
The present invention relates to a method of and a system for controlling frame synchronization, and more particularly to a method of and a system for controlling frame synchronization for European Digital Audio Broadcasting (DAB), which are capable of quickly controlling frame synchronization.
2. Description of the Related Art
DAB is an audio broadcasting service capable of transmitting characters, graphic images, and motion pictures as well as high quality audio and voice sound such as audio compact disc (CD) sound beyond typical audio sound such as AM and FM. The DAB provides a terrestrial broadcasting, which is offered for free in a local area, and a satellite DAB, which usually provides pay-for-use multimedia broadcasting service and is widely provided using both satellites and terrestrial networks. The DAB using ground waves has already been generally provided as a service in Europe and the United States of America (U.S.). For the standard of the DAB, the U.S. adopts an In Band on Channel scheme but Europe adopts Eureka 147 which is an Out of Band scheme. In Europe, the DAB started to be developed in 1987 and the European Standard for the DAB has been established since 1995 since the United Kingdom launched the service in 1995. Further, the DAB service is now being provided in France, Germany and Sweden.
Contrasting the European scheme with the U.S. scheme, the DAB in the U.S. uses both of analog broadcasting and digital broadcasting by converting the analog broadcasting to digital broadcasting. Meanwhile, the DAB in Europe is a new concept of service where new service providers and existing service providers enter the market together. Korea established the Eureka 147 of Europe as its national standard, and launched the terrestrial DAB service over the metropolitan area starting in 2004. In order to enable European DAB receivers to operate normally, at least synchronization should be performed. Synchronization of the European DAB receivers is carried out by performing frame synchronization, frequency synchronization and symbol synchronization, in turn.
FIG. 1 illustrates a composition of a frame. A frame is comprised of a set of symbols, 0 to 76. Orthogonal frequency division multiplexing (OFDM) symbols constitute a DAB frame and the maximum value thereof changes depending on the mode of a symbol counter and a null symbol always has a value zero. Meanwhile, each OFDM symbol includes a set of samples, 0 to 2551.
To implement the frame synchronization, first, a beginning point of a DAB frame should be looked for. The frame synchronization comprises a frame offset search period for estimating a frame offset value and a frame offset compensation period for applying the estimated frame offset value. Generally, the search period is longer than a time corresponding to the sum of an interval of at least one frame and an interval of the null symbols. For the search period, a power-moving average scheme typically using two windows is applied.
FIG. 2 is a conceptual diagram of the power-moving average scheme. The scheme is based on a time and is a method for searching an interval of one or more frames using two windows 230, 260 with the length L. For each sample, a beginning point 290 of a frame is determined at a point having the maximum energy ratio of the two windows 230, 260. Equation 1 expresses the method of searching the beginning point 290 in the power-moving average scheme.
                              Frame          start_point                =                                            MAX              i                        ⁢                                          energy                ⁡                                  (                                      window                    ⁢                                                                                  ⁢                    2                    ⁢                                          (                      i                      )                                                        )                                                            energy                ⁡                                  (                                      window                    ⁢                                                                                  ⁢                    1                    ⁢                                          (                      i                      )                                                        )                                                              -          L                                    Equation        ⁢                                  ⁢        1            
FIG. 3 illustrates a constitution of a frame control system. A DAB receiver should have at least a frame control unit 300 and a frame synchronization unit 350 for its normal operation. The frame control unit 300 is an important controlling part for the receiver, and operates by being synchronized with a transmitted DAB frame signal after receiving a frame offset and a symbol offset. Each block in the receiver performs its own function by receiving control signals from the frame control unit 300. The frame synchronization unit 350 estimates a frame offset value during the frame offset search period described above and inputs the frame offset value to the frame control unit 300. That is, the frame control unit 300 operates by receiving the frame offset value and the control signals from the frame synchronization unit 350. Here, the frame control unit 300 and the frame synchronization unit 350 operate independently and the frame offset value is updated by a frame.
FIG. 4 illustrates a method of controlling frame synchronization in accordance with a related art. A method of controlling the frame synchronization in accordance with a related art will be described below with reference to FIG. 3 and FIG. 4. When a power is applied to the DAB receiver, the frame control unit 300 starts symbol counting. With reference to FIG. 4, reference numeral 450 denotes a symbol counting clock signal, pulses of the symbol counting clock signal 450 are not coincident with pulses of an external clock signal 400, so that the frame synchronization is needed. Meanwhile, the frame synchronization unit 350 estimates a frame offset value 420 during the search period 410. The frame synchronization unit 350 inputs the estimated offset value 420 into the frame control unit 300, and the frame control unit 300 counts up the null symbols as many as the number of times which correspond to the offset value 420 when counting the null symbols during a symbol counting period. As a result, the DAB receiver can normally operate.
However, in the method in accordance with the related art referenced in FIG. 4, there occurs a delay time corresponding to 3 frames (average 2.5 frames) between estimation of the frame offset value and application of the estimated frame offset value. That is why the frame synchronization unit 350 and the frame control unit 300 operate independently. That is, the delay time corresponding to at least one frame occurs after estimating the offset value. This affects the startup time of a receiver, thereby increasing the total startup time and delaying operations of under-layer logic elements.